Nanocomposite Material Potentially Useful for Solar Energy, other Energy Technologies
Two
nanotech methods for engineering solar cell materials have shown
particular promise. One uses thin films of metal oxide nanoparticles,
such as titanium dioxide, doped with other elements, such as nitrogen.
Another strategy employs quantum dots--nanosize crystals--that strongly
absorb visible light. These tiny semiconductors inject electrons into a
metal oxide film, or "sensitize" it, to increase solar energy
conversion. Both doping and quantum dot sensitization extend the
visible light absorption of the metal oxide materials.
Combining these two approaches appears to yield better solar cell
materials than either one alone does, according to Jin Zhang, professor
of chemistry at the University of California, Santa Cruz. Zhang led a
team of researchers from California, Mexico, and China that created a
thin film doped with nitrogen and sensitized with quantum dots. When
tested, the new nanocomposite material performed better than predicted
-- as if the functioning of the whole material was greater than the sum
of its two individual components.
"We have discovered a new strategy that could be very useful for
enhancing the photo response and conversion efficiency of solar cells
based on nanomaterials," Zhang said. "We initially thought that the
best we might do is get results as good as the sum of the two, and
maybe if we didn't make this right, we'd get something worse. But
surprisingly, these materials were much better."
The group's findings were reported in the Journal of Physical Chemistry
in a paper posted online on Jan. 4. Lead author of the paper was
Tzarara Lopez-Luke, a graduate student visiting in Zheng's lab who is
now at the Instituto de Investigaciones Metalurgicas, UMSNH, Morelia,
Mexico.
Zhang's team characterized the new nanocomposite material using a
broad range of tools, including atomic force microscopy (AFM),
transmission electron microscopy (TEM), Raman spectroscopy and
photoelectrochemistry techniques. They prepared films with thicknesses
between 150 and 1100 nanometers, with titanium dioxide particles that
had an average size of 100 nanometers. They doped the titanium dioxide
lattice with nitrogen atoms. To this thin film, they chemically linked
quantum dots made of cadmium selenide for sensitization.
The resulting hybrid material offered a combination of advantages.
Nitrogen doping allowed the material to absorb a broad range of light
energy, including energy from the visible region of the electromagnetic
spectrum. The quantum dots also enhanced visible light absorption and
boosted the photocurrent and power conversion of the material.
When compared with materials that were just doped with nitrogen or
just embedded with cadmium selenide quantum dots, the nanocomposite
showed higher performance, as measured by the "incident photon to
current conversion efficiency" (IPCE), the team reported. The
nanocomposite's IPCE was as much as three times greater than the sum of
the IPCEs for the two other materials, Zhang said.
The nanocomposite material could be used not only to enhance solar
cells, but also to serve as part of other energy technologies. One of
Zhang's long-term goals is to marry a highly efficient solar cell with
a state-of-the-art photoelectrochemical cell. Such a device could, in
theory, use energy generated from sunlight to split water and produce
hydrogen fuel (see http://press.ucsc.edu/text.asp?pid=712).
The nanocomposite material could also potentially be useful in devices
for converting carbon dioxide into hydrocarbon fuels, such as methane.